Complex-shaped forged piston oil galleries

ABSTRACT

A steel piston with an oil gallery, and process for forming a steel piston oil gallery channel, which corresponds to the complex shape of the combustion bowl in the piston crown. The oil gallery channel is first forged to the basic shape that corresponds to the shape of the walls of the combustion bowl. Machine-turning surfaces in the oil gallery channel can be machine-finished as desired. Surfaces in the oil gallery which cannot be machined with conventional turning operations, such as recesses and protrusions into the channel, are left in the original forged condition.

CROSS-REFERENCE TO CO-PENDING APPLICATION

This U.S. Continuation Application claims the benefit of U.S.Continuation application Ser. No. 15/096,498, filed Apr. 12, 2016 whichclaims the benefit of U.S. Utility application Ser. No. 13/769,751,filed Feb. 18, 2013, now U.S. Pat. No. 9,334,958, issued May 10, 2016which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The invention relates generally to pistons for internal combustionengines, particularly diesel engines, and more particularly to pistonshaving oil galleries.

BACKGROUND

It is known in internal combustion engine applications, particularlywith respect to diesel engines, to provide pistons with bodies formedwith a closed gallery for cooling oil. The oil circulates through thegallery and cools parts of the piston which are susceptible to damagefrom the heat of combustion. The upper rims around the combustion bowlsare particularly susceptible to damage when overheated.

The cooling galleries are generally annular or ring-shaped with constantcross-sections and are provided inside the piston ring belt and adjacentthe top wall and rim of the piston body. The gallery is bounded by aninner wall adjacent the combustion bowl. The oil galleries can be eitheropen or closed. If closed, the gallery channel is closed at the bottomby a bottom wall. For closed galleries, inlets can be provided in thebottom wall for receiving cooling oil into the gallery, or the coolingoil could be supplied to the galleries by other means.

Traditionally, the combustion bowls of diesel engines have circularsymmetric shapes and have smooth unbroken surfaces from the outer rimsto the depressed center portions. Today, however, new bowls are beingdesigned with non-traditional or complex shapes. It is understood thatthese non-traditional shapes are utilized in order to burn fuel moreeffectively and with less undesirable emissions. However, the complexbowl shapes makes the cooling of the bowls and rims with conventionalpiston galleries more difficult. Conventional machining with turningoperations will not provide oil galleries with similar or correspondingcomplex shapes as the complex shaped combustion bowls.

If the differences in structure between the combustion bowls and the oilgalleries create walls with significantly different thicknesses, orwalls which are too thick to be adequately cooled by the oil beingcirculated in the oil galleries, then “hot spots” in the piston canoccur where the material can overheat. Hot spot areas can create weakspots in the piston where the material could crack or fail. If a pistonfails, this creates an engine failure causing major expense and perhapsa new engine for the vehicle.

It is an object of the present invention to provide a forged steel oilgallery, and a method of forming a forged steel oil gallery, whichaccommodates non-traditional complex-shaped combustion bowls andmaintains thin and uniform wall thickness in order to avoid overheatingconcerns.

SUMMARY OF THE INVENTION

The invention provides oil galleries for forged steel pistons withcomplex shaped combustion bowls, as well as methods for forming such oilgalleries, which provide substantially uniform wall thickness betweenthe combustion bowls and the oil galleries. The invention minimizes oreliminates hot spots and allows the oil in the galleries to maintain therims and other areas of the pistons and combustion bowls withinacceptable temperature limits.

The galleries are formed in the same or substantially similar complexshapes as the combustion bowls. The galleries are formed by forging andthen machined by conventional turning operations where possible toenlarge the galleries and to finish certain surfaces. Areas and surfacesin the galleries which cannot be finished by conventionally machineturning operations, such as recesses and bulges, are left in theirinitial forged condition. During the machine turning, an annular grooveis formed extending the shape of the oil gallery in a direction towardthe bowl rim.

The term “complex” as used herein refers to the shape of the combustionbowl in the piston crown which is not traditionally shaped, either inits outer perimeter, or inside the outer perimeter, or both. “Complex”shapes refers to all shapes of a combustion bowl other than traditionaland which can have, for example, edges which include straight, curved,or arced sections, or which have bumps, protrusions, ribs, recesses andthe like either in the bowl or, its outer perimeter, or both. Ingeneral, complex shapes are any shapes which are not machinable byconventional machine-turning operations.

The present invention is preferably utilized for pistons for dieselengines, although the invention can also be utilized for pistons for anyinternal combustion engine and for engines that use either liquid or gasfuel.

Once the crown of the piston if formed, the crown is attached to asecond portion of the piston in order to complete the full piston. Thesecond portion typically includes the remainder of the side walls andthe pin bores. The crown and second portion are connected together inany known manner, but preferably by friction welding. A connectionprocess of this type is disclosed, for example, in U.S. Pat. No.6,260,472.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more readily appreciated when considered in connection with thefollowing detailed description and appending drawings, wherein:

FIG. 1 illustrates a piston with a closed oil gallery.

FIGS. 2A, 2B and 2C schematically illustrate three general types ofcomplex combustion bowls.

FIG. 3 is a schematic plan view of a piston crown depicting arepresentative complex shape of a combustion bowl.

FIG. 4 is a cross-section of the piston crown as shown in FIG. 1 takenalong line 4-4 and in direction of the arrows.

FIG. 5 is another cross-section of the piston crown similar to FIG. 4,following machining-turning of at least one surface of the oil gallery.

FIG. 6 is a flow chart of an embodiment of a system and method inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A representative piston 100 in which the present invention can beutilized if the combustion bowl has a complex shape is shown in FIG. 1.The piston 100 includes a piston crown member 10 and a lower member 102which includes sidewalls 104 and pin bosses 106. The piston crown 10 andlower member 102 are fixedly secured together, preferably by frictionwelding, to form the complete piston 100.

The piston 100 has an oil gallery in which oil is circulated in order tomaintain the temperature of the piston, particularly the upper surface,combustion bowl and outer rim within acceptable temperature limits. Theoil gallery includes an oil gallery channel 105 positioned in the crownmember 10. The oil gallery can be either opened or closed as well knownin the art. If closed, the bottom wall of the oil gallery is typicallyincluded as part of the lower member 102.

A piston crown member 10 with a representative complex combustion bowl12 is depicted in FIG. 3. The shape of the bowl 12 is essentially asquare shape with four sides 15, 16, 17 and 18. In the shape depicted,the sides are straight with rounded corners 21, 22, 23 and 24.

It is to be understood that the shape of the combustion bowl and thelinearity of the sides 15-18 is merely one example of a complexcombustion bowl. In accordance with the invention, the combustion bowlcan have any perimeter or internal shape, with any number of sides orside portions. The sides and internal surfaces can also have any shape,such as being straight as shown or curved or arced, and can havesections or portions which are straight, curved or arced, or have bumps,protrusions, recesses, ribs and the like. In addition, the corners21-24, or the intersections or joints between the side portions, canhave any shape, and can protrude into the bowl or be recessed from it.

The shape of the piston bowl can be complex either in its outerperiphery, as shown in FIG. 2A, be complex in the radially inner areasof the bowl, as shown in FIG. 2B, or be complex with respect to both theouter periphery and the inner areas as shown in FIG. 2C. FIGS. 2A, 2Band 2C are representative of these three general types of complex shapedcombustion bowls. The present invention provides an oil gallery and oilgallery channel that can accommodate combustion bowls with such complexshapes.

The piston crown and the entire piston are made of a steel material. Theshape of the piston crown 10 is formed by a forging process.

In accordance with a preferred embodiment of the invention, the oilgallery channel in the piston crown is made by the same process as thecombustion bowl, or portions thereof. In the embodiment shown, the oilgallery channel is made by a forging process, followed by a machiningprocess.

In accordance with a preferred embodiment, the forging die for formingthe combustion bowl and the forging die for forming the oil gallerychannel have corresponding shapes. The two dies have similar straightsections and similar curved sections that correspond to one another.

A cross-section of the piston crown 10 after the forging process isshown in FIG. 4. The forging process forms an annular groove 30 thatdoes not necessarily have a circumferential uniform width “W” or auniform depth “D”. The width of the groove 30 is greater in the portionswhere the sides of the combustion bowl are furthest from the outsidesurface of the piston crown. These areas are designated by the numeral40 in FIG. 3. Similarly, the areas where the groove 30 is the narrowestis at the corners or intersections between the side portions.

The depth D of the groove 30 made by the forging is dependent on thedies used in the forging process. There is a practical limit to thedepth that forging dies can penetrate in a steel crown member and stillbe used repeatedly before they need replacing or refurbishing.

As a subsequent step in forming the oil galleries in accordance with apreferred embodiment of the invention, the shape of the oil gallery ismachined to the shape shown in FIG. 5. In one step in the machineprocessing, a machining tool represented by the member 50 is insertedinto the oil gallery 30 formed by the forging process (in the directionof arrow 52) and used to finish the outer surface of the oil gallerychannel and to form an annular groove 60, entirely around the bowl. Thismachine-turning extends the oil gallery channel into the upper reachesof the piston crown (near the top ring groove and adjacent the top faceor rim 80 of the piston). The groove 60, which is fully machined,extends above the initial channel 30 made in the forging process.

“Machine-turning” or simply “turning” is a machining process in which acutting tool, typically a non-rotary tool bit, moves linearly while theworkpiece rotates, such as on a lathe. “Maching turning” can refer tosuch a cutting or finishing operation on either the internal surfaces orthe external surfaces of a workpiece. In machining some of the surfacesof an oil gallery channel thereon, the machine-turning finishes or formsinternal surfaces.

The machine-turning processing can also be used to machine and finishsome of the inner surfaces of the oil gallery channel 30, such assurface 31. Due to the complex shape in the oil gallery in order tofollow the complex shape of the combustion bowl, a plurality of bumps orrecesses can be formed on the inside gallery surface by the forging diein the forging process. Due to the turning procedure used in themachining process, the recesses (or pocket portions) and areas betweenbumps are left unfinished (i.e. not machined) in this step. Innergallery channel surfaces 32 and 33 in FIG. 5 are not finished and remainin their original condition after forging.

In FIG. 3, the outer circumference of the finished annular groove 60 isindicated by hidden line 60′. In addition, an inner finished surface ofthe complex shaped oil gallery (such as surface 31) is indicated by thehidden line 31. The areas (e.g. recesses) left unfinished are indicatedby the reference numeral 32 in FIG. 3 and numerals 32 and 33 in FIG. 5.An oil gallery channel having the shape 30 shown in FIG. 4 can be formedby a forging die.

In a typical forging operation, the oil gallery channel in the lowersurface of the piston crown member 10 will be formed at the same timethat another forging die is forming the complex shaped combustion bowlon the upper surface or upper side of the piston crown member.

The inventive process provides an annular oil gallery for a piston whichhas a similar or substantially the same perimeter inner shape as theouter perimeter shape of a complex-shaped combustion bowl. Thisminimizes the thicknesses of the wall areas 70 between the oil galleryand combustion bowl 12 and makes the wall thicknesses around the outsideof the combustion bowl uniform. Due to practical limits of the forgingand machining processes, the thicknesses of all of the walls will not beexactly the same around the circumference of the combustion bowl. Thepresent invention, however, makes the wall thickness 70 as thin anduniform as practical around the entire oil gallery. This allows oilintroduced in the oil gallery when the piston is finished to maintainthe temperature of the bowl wall surfaces and rim 80 within appropriatelimits and avoids harmful hot spots.

With the present invention, thick wall sections which can create hotspots are minimized or eliminated. Areas of a piston which overheat(also called “hot spots”) can create weak spots that could crack orfail. Failure of pistons in this manner could lead to expensive repairs,and possibly to the replacement of the engine.

Preferred cooling for a piston can be provided when the walls have auniform thickness and are relatively thin. Pistons with thinner wallsalso have less weight which provides less strain on the engine. Thisresults in better gas mileage and less harmful emissions.

FIG. 6 presents a flow chart 108 of the preferred method of forming anoil gallery channel in a piston crown that corresponds in shape to theshape of a complex combustion bowl in the piston crown. First, a pistoncrown member is made of a steel material 110. This can be done by anyconventional manufacturing procedure.

Then, a complex shaped combustion bowl is formed in the top or uppersurface of the piston crown member 112. This step is formed by a steelforging process.

At the same time or thereafter, an oil gallery channel 30 is formed inthe lower surface or lower side of the piston crown member with a shapethat is similar to, or corresponds to, the shape of the combustion bowl.This is shown in box 114. This step is also performed by a forgingprocess, and preferably simultaneously with forging the bowl.

Finally, the oil gallery channel in the piston crown member is machinefinished 116 to enlarge its size and bring it closer to the upper rim ofthe piston crown member and the combustion bowl. In this step, anannular groove 60 is formed by a cutting tool, and the other surfaces ofthe gallery channel which can be finished by a machine-turning operation(i.e. other than recesses and areas between bumps) can be finished asdesired.

Although the invention has been described with respect to preferredembodiments, it is to be also understood that it is not to be so limitedsince changes and modifications can be made therein which are within thefull scope of this invention as detailed by the following claims.

What is claimed is:
 1. A piston for an internal combustion engine including at least one pin boss, at least one piston skirt portion; a combustion bowl recessed in an upper surface of said piston body, and an oil gallery below said combustion bowl, and wherein said oil gallery has a radially inner wall surface with portions thereof that are machined and recessed portions that are unmachined and in an as-forged condition.
 2. The piston of claim 1 wherein the machined portions are adjacent the as-forged portions.
 3. The piston of claim 1 wherein the oil gallery includes a bottom wall.
 4. The piston of claim 3 wherein the bottom wall includes at least one opening for the ingress or egress of oil into and/or out of the oil gallery.
 5. The piston of claim 1 wherein the piston is made of two pieces joined together.
 6. The piston of claim 1 wherein the combustion bowl has a complex shape and the inner wall surface of the oil gallery at least in part complements the shape of the combustion bowl. 